This invention relates to a method and apparatus for marking glass with a laser, particularly in a tube drawing installation.
Various methods are known from the prior art for marking or inscribing glass by means of laser radiation:
A method of producing a body from transparent material with a mark in the inside of the body, is known from DE 44 07 547 C2, wherein there is a limited space in the interior of the body in which point-like microcracks are formed by laser radiation. The microcracks have a diameter such that that they are visible to the naked eye.
WO 92/03297 A1 (=DE 41 266 26 C2) relates to a similar method, wherein the material in the interior of the glass body to be marked is ionized by laser radiation, so that microcracks are likewise formed.
Another similar method is known from WO 94/114567, wherein an image is generated in the interior of the body by the formation of local microcracks.
The aforementioned methods which are known from the prior art employ solid-state lasers (Nd:YAG) with a high energy density of  greater than 107 W/cm2. A disadvantage of the aforementioned methods is that it is only certain glasses which react with laser radiation of this type. Another disadvantage is that the microcrack structure impairs the material properties of the marked glass.
Methods of marking glass which also employ solid lasers (Nd:YAG) are known from WO 00/32349 A1 and WO 00/32531 A1, wherein the laser parameters are selected so no fracture-inducing microcracks are formed. A disadvantage of these methods, however, is that only marks which are invisible to the naked eye can be produced. A further disadvantage is that focusing the laser-beam in the interior of the glass body presupposes a sufficiently large volume of glass or a minimum wall thickness of at least 1 mm, for example, in order to prevent crack growth as far as the surface.
In order to achieve an interaction or a marking effect despite the extensive transparency of glass to the wavelengths from solid-state lasers, it is known that absorbent layers can be applied to the glass. A method such as this is known from EP 07 613 77 B1, wherein a Nd:YAG laser with a wavelength of 1.06 xcexcm, for example, is used for marking the applied layer of material. Furthermore, a corresponding method, wherein a metal-doped special glass is marked by an Nd:YAG laser, is known from DE 422 428 2 A1.
A method for internally marking a glass body without changing the glass surface is known from WO 95/05286 A1. For this purpose, a CO2 laser is used which has an energy density of at least 6 kW/cm2 at its focus, in order to introduce local stresses under the surface to a depth of about 50 xcexcm. A disadvantage of this method is that the marks are not visible to the naked eye.
Moreover, a method of decorating glass products is known from DE 312 1138 C2, wherein a laser beam is employed which has a maximum energy such that glass material is evaporated from the surface layer or such that a change in the optical transparency of the glass is produced.
Another method of removing material from a glass surface by means of a laser beam is known from DE 314 5278 C2, wherein the laser beam is subdivided into a multiplicity of individual beams by a partially absorbing matrix.
Furthermore, a method wherein the glass is melted by a laser beam at certain points on the surface to be treated is known from the DE 413 2817 A1. This material removal method has the disadvantage that extraction devices are necessary in order to remove volatilised glass material. Moreover, there is a deleterious effect on the properties of the material.
One particular disadvantage of the aforementioned methods is the requisite heat treatment of the glass before, during or after laser treatment in order to relieve the glass from stress, since laser treatment takes place at temperatures below the transformation temperature.
A method of engraving glass vessels by means of laser beams is known from DE 199 26878 A1. This method employs an enamel which consists of a combination of a vitrifiable base with a pigment which has the property of reacting under the action of the laser beam and of changing the color of the combination used.
In addition, a method is known from WO 99/00215 A1 which is based on a combination of known methods for the surface marking and internal marking of glass.
A method of the marking a glass surface by means of UV laser radiation is known from WO 96/10777, but this only results in a microstructure which cannot be recognized without visual assistance.
A method of marking a glass substrate is known from JP 09 278494 A. A YLF laser with a wavelength of about 262 nm is used for marking.
Another method of marking glass is known from JP 10 101379 A. This method employs pulsed laser beams with a wavelength of 2300 nm, wherein each location which is to be marked on the glass is acted upon between three and one hundred times by a laser beam.
A method of providing a body of material made of glass or plastics with a mark situated under the surface is known from DE 41 26 626 C2. For this purpose, a beam of high energy density, to which the material is permeable, is directed toward the surface of the body of material. The beam is focused at a point which is at a distance from the surface and which is disposed inside the body of material, whereby a mark is produced.
A method of inscribing information within the bulk of transpatent materials by means of a laser beam is known from DE 34 25 263 A1, wherein the information can be inscribed at various depths within the material by the choice of focusing of the laser beam.
A method of producing a defined, durable change, by means of intensive laser pulses, in the extinction spectrum of dielectric materials which contain metal particles is known from DE 198 23 257 A1. The change is produced by irradiating an extinction band caused by the excitation of surface plasmons in the metal particles with intensive laser light.
It is therefore an object of the present invention to create an improved method and an improved apparatus for marking glass by means of a laser.
The invention provides for a method of marking glass whereby a laser pulse is used to apply a mark to a surface of the glass at a position where the glass temperature is above the transformation temperature of the glass. Further the invention provides for an apparatus for marking glass, in particular a tube drawing installation, for performing the method of the invention.
The invention enables the marking of glass to be incorporated into the production process, for example in a tube drawing installation. The need for separate processing steps for the application of marks after the production of the glass is thereby dispensed with.
Furthermore, the invention enables a mark, which is clearly visible but which is free from micro-cracks, to be applied to the glass even at high temperatures during the manufacture of the tube. The possible incorporation of a marking operation into the production process enables the high temperatures which exist anyway to be used for the application of the method according to the invention, without these temperatures, which are higher than the transformation temperature of the glass, having to be generated in subsequent processing steps.
One particular advantage of the invention is that it can be included in a continuous production process without delaying or otherwise adversely affecting the latter.
Another advantage is that the invention enables low-stress marks to be produced on the surface without microcracks, so that the material properties of the manufactured glass are not disadvantageously affected by the marking process. In a preferred embodiment, the laser beam is controlled so that it reaches its peak power for a short time only. In this respect, the peak power is selected depending on the physical parameters of the glass, for example its thermal expansion coefficient and thermal conductivity, in order to minimize the introduction of stress into the glass.
Preferably the laser power is set to a level where the laser pulse which impinges upon the glass surface which is already above its transformation temperature momentarily melts a small amount of glass on the glass surface to form the mark without evaporating a substantial amount of glass. This purely thermal interaction is sufficient for the application of visible marks to the glass surface, since the glass tobe marked is at a temperature above the transformation temperature, and the glass is therefore more easily workable. Accordingly, only a relatively low laser power is necessary for the application of marks.
In a preferred embodiment, a pulsed quasi-continuous CO2 laser beam is employed. By employing different pulse/spacing ratios of the laser beam, any durable marks of different patterns, for example points, strokes or lines, can be produced on the glass surface which moves past the laser.
The control system for generating the laser beam can advantageously be connected to measuring instruments. This enables the glass to be marked corresponding to defects which have been identified by measuring devices for quality assurance. For example, if a measuring device determines defects in the glass, e.g. bubbles or protuberances, the measuring device can send a corresponding signal to the control electronics of the laser. As a result, a mark is applied to the surface of the relevant region of the material which can either be used subsequently in the production process to indicate any post-treatment which is necessary or to reject the corresponding product.
The pulse times and/or peak power levels of the laser beam can also be selected so that the marks cannot be discerned by the naked eye but can only be detected under the microscope or by means of a polarimeter. The marks can also be formed so that they are machine-readable, in the form of what is termed a bar code, for example.
With the method according to the invention, marks which cannot be counterfeited can be applied to the glass, for example, in order to combat product piracy. Likewise, trade marks, company logos or other product requisites can be applied to the glass by the method according to the invention so that they cannot be counterfeited, without this having an adverse effect on the material properties of the glass and with complete integration of the application of the mark into the production operation.
Since the method operates without contact, the corresponding apparatus can be constructed so that it can be encased separately from the remainder of the production process. This has the further advantages of ensuring a low maintenance requirement and of enabling a marking unit of simple, mobile design to be employed.